1. Field of the Invention
The field of the invention is an optical system having apparatus for compensating for differences in reflectivity between two or more reflecting surfaces which reflect the same light to a photodetector.
2. Description of the Prior Art
Heretofore in dual wavelength spectrophotometers where one of two light beams is intermittently reflected by a chopper mirror toward and through a fluid sample to a photodetector, errors are incurred in the measurement of transmittance of light through the sample by reason of the different reflectivities of the mirror segments.
Manufacturers of such spectrophotometers have relied on the supplier of the chopper mirror to match reflectivity among the various mirror segments. With the best matches provided by the suppliers, a difference in reflectivity of 1% is still incurred.
Ideally, the mirror segments should have equal reflectivities so that the electrical signal level produced by a collimated light source reflected by a particular mirror segment and detected by the photodetector will have the same signal level regardless of which segment was reflecting the light to the photodetector. However, manufacturing tolerances result in reflectivity differences among segments of approximately 1 to 2%. These differences produce an undesirable variation in the electrical signal output from the photodetector and errors in the measurement of transmittance of light through a sample situated in the path of the light from the reflecting surface to the photodetector.
As will be described in greater detail hereinafter, the reflectivity compensating apparatus of the present invention provides a sensing device for identifying which of the segments is currently reflecting light from the light source to the photodetector and potentiometers for attenuating the resulting signal levels to a base line signal level produced by the segment which has the lowest reflectivity.